PCLOSE

NAME

SYNOPSIS

DESCRIPTION

The pclose() function shall close a stream that was opened by
popen(), wait
for the command to terminate, and return the termination status of
the process that was running the command language interpreter.
However, if a call caused the termination status to be unavailable
to pclose(), then pclose() shall return -1 with
errno set to [ECHILD] to report this situation. This can happen
if the application calls one of the following functions:

*

wait()

*

waitpid() with a pid argument less than or equal to 0
or equal to the
process ID of the command line interpreter

*

Any other function not defined in this volume of IEEE Std 1003.1-2001
that could do one of the above

In any case, pclose() shall not return before the child process
created by popen() has terminated.

If the command language interpreter cannot be executed, the child
termination status returned by pclose() shall be as if
the command language interpreter terminated using exit(127)
or _exit(127).

The pclose() function shall not affect the termination status
of any child of the calling process other than the one
created by popen() for the associated stream.

If the argument stream to pclose() is not a pointer to
a stream created by popen(), the result of pclose() is
undefined.

RETURN VALUE

Upon successful return, pclose() shall return the termination
status of the command language interpreter. Otherwise,
pclose() shall return -1 and set errno to indicate the
error.

ERRORS

The pclose() function shall fail if:

ECHILD

The status of the child process could not be obtained, as described
above.

The following sections are informative.

EXAMPLES

None.

APPLICATION USAGE

None.

RATIONALE

There is a requirement that pclose() not return before the child
process terminates. This is intended to disallow
implementations that return [EINTR] if a signal is received while
waiting. If pclose() returned before the child terminated,
there would be no way for the application to discover which child
used to be associated with the stream, and it could not do the
cleanup itself.

If the stream pointed to by stream was not created by popen(),
historical
implementations of pclose() return -1 without setting errno.
To avoid requiring pclose() to set errno
in this case, IEEE Std 1003.1-2001 makes the behavior unspecified.
An application should not use pclose() to close
any stream that was not created by popen().

Some historical implementations of pclose() either block or
ignore the signals SIGINT, SIGQUIT, and SIGHUP while waiting
for the child process to terminate. Since this behavior is not described
for the pclose() function in
IEEE Std 1003.1-2001, such implementations are not conforming. Also,
some historical implementations return [EINTR] if a
signal is received, even though the child process has not terminated.
Such implementations are also considered non-conforming.

Consider, for example, an application that uses:

popen("command", "r")

to start command, which is part of the same application. The
parent writes a prompt to its standard output (presumably
the terminal) and then reads from the popen()ed stream. The
child reads the response
from the user, does some transformation on the response (pathname
expansion, perhaps) and writes the result to its standard output.
The parent process reads the result from the pipe, does something
with it, and prints another prompt. The cycle repeats. Assuming
that both processes do appropriate buffer flushing, this would be
expected to work.

To conform to IEEE Std 1003.1-2001, pclose() must use waitpid(),
or some similar function, instead of wait().

The code sample below illustrates how the pclose() function
might be implemented on a system conforming to
IEEE Std 1003.1-2001.

FUTURE DIRECTIONS

SEE ALSO

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .